Na, K -ATPase in the myocardial cell membrane is essential for the generation and maintenance of transmembrane electro-chemical gradients and hence for the maintenance of excitability of cardiac cells. The consequences of the inhibition of this enzyme system on the function of the heart are not well understood. Cardiac glycosides inhibit myocardial Na ions, K ions-ATPase and simultaneously increase contractile force. It appears that the enzyme inhibition by cardiac glycosides causes an enhanced intracellular sodium transient associated with membrane excitation, resulting in an enhanced calcium transient and increased contractility. The causal relationship betwen the enzyme inhibition and inotropic response, however, is not firmly established. The objective of this project is to determine whether there is a causal relationship between the inhibition of cardiac Na ions, K ions-ATPase by cardiac glycosides and the inotropic and/or toxic effect of these drugs. This will be achieved by studying the relationship between sodium pump inhibition (as monitored by ouabain-sensitive 86Rb transport) and inotropic response in isolated heart preparations during the development and dissipation of the inotropic response to various Na ions, Kions-ATPase inhibitors including cardiac glycosides. Further, the consequences of sodium pump inhibition which lead to the positive inotropic and/or toxic responses will be studied in order to understand the molecular mechanism of action of cardiac glycosides. The nature of the molecular interaction between the cardiac glycosides and their inotropic and/or toxic (arrhythmic) "receptor" and factors which influence this interaction wll also be studied and compared with those between cardiac glycosides and isolated Na ions, K ions-ATPase. Finally, the differences in the sequence of biochemical events which link Na ions, K ions-ATPase inhibition to inotropic or toxic responses, respectively, will be explored.